1. Field of the Invention
The present invention generally relates to a pixel circuit and display apparatus having a light emitting element driven by active matrix method, and more particularly to a pixel circuit using an inorganic oxide thin film transistor.
2. Description of the Related Art
Display devices using light emitting elements, such as organic EL element and the like, are proposed for use in various fields including televisions, cell phone displays, and the like.
Generally, the organic EL element is a current-driven type light emitting element, thus pixel circuits including an organic EL element proposed have a configuration like that shown in FIG. 9 as described, for example, in U.S. Pat. No. 5,684,365.
The pixel circuit shown in FIG. 9 includes switching transistor 104, capacitor element 103, and driving transistor 102 as a minimum configuration. In the configuration, when switching transistor 104 is turned ON, a program signal, which will serve as a gate voltage of driving transistor 102, is written in capacitor element 103, and the gate voltage according to the program signal is applied to driving transistor 102 so as to perform constant current operation, whereby a drive current flows through organic EL element 101 and light is emitted from the device.
In conventional pixel circuits, low-temperature polysilicon or amorphous silicon thin film transistors are used as the switching transistor and driving transistor.
The low-temperature polysilicon thin film transistor may provide high mobility and high stability of threshold voltage, but has a problem that the mobility is not uniform. The amorphous silicon thin film transistor may provide uniform mobility, but has a problem that the mobility is low and threshold voltage varies with time. The non-uniform mobility and instable threshold voltage appear as irregularities in the display image.
Consequently, Japanese Unexamined Patent Publication No. 2003-255856 proposes a pixel circuit having therein a compensation circuit for correcting the threshold voltage.
The provision of the compensation circuit, however, causes the pixel circuit to become complicated, resulting in increased cost due to low yield rate and low aperture ratio.
As such, thin film transistors made of inorganic oxide films, as typified by IGZO, have recently been drawing attention. The thin film transistors made of inorganic oxide films allow low-temperature film forming and have features of providing sufficient mobility, highly uniform mobility, and low threshold voltage variation with time.
Where thin film transistors are fabricated with inorganic oxide films in order to obtain various desired characteristics and when trying to obtain desired current characteristics, however, the threshold voltage that causes the transistors to perform OFF operation may sometimes become a negative voltage.
For example, when trying to control a thin film transistor, used as the driving transistor whose OFF-operation threshold voltage is a negative voltage like that described, for example, “Highly Stable Ga2O3—In2O3—ZnO TFT for Active-Matrix Organic Light-Emitting Diode Display Application”, C. J. Kim et al., IEDM (International Electron Device Meeting) 2006, Samsung Advanced Institute of Technology (Non-Patent Document 1) by the data driving circuit of a conventional organic EL display device, the minimum setup value of the gate voltage of the driving transistor of the conventional data driving circuit is 0 v, so that a minimum drive current, which is the value when gate-source voltage VGS of the driving transistor is 0 v, flows through the organic EL element, thus unable to cause the EL element to stop the emission.
FIG. 10 shows voltage waveforms of scan signal, data signal, and gate-source voltage VGS of driving transistor 102 when the thin film transistor described in Non-Patent document 1 is used in the pixel circuit shown in FIG. 9.
Use of a thin film transistor whose OFF-operation threshold voltage is a negative voltage as driving transistor 102 results in that driving transistor 102 is unable to perform OFF operation as shown in FIG. 10, therefore unable to cause organic EL element to stop the emission, causing difficulties in emission control in a low brightness region.
In order to solve the problems described above, it is conceivable to provide a voltage source to set the ground wire of the pixel circuit at a voltage (VA) higher than 0 v, as shown in FIG. 10. But this method greatly increases power consumption of the display device as a whole, whereby the feature of low power consumption of EL element is spoiled.
It is also conceivable to set the ground wire of the data drive circuit that supplies a program signal at a voltage higher than 0 v, thereby causing the program signal to become negative. But in order to ensure the data connection level with an external device, it is necessary to newly develop a dedicated IC, which becomes a cost increase factor of the display device.
In view of the circumstances described above, it is an object of the present invention to provide a pixel circuit that uses an inorganic oxide thin film transistor whose OFF-operation threshold voltage is a negative voltage, yet does not increase power consumption and allows the use of a conventional drive circuit, and a display apparatus that uses the pixel circuit.